The present invention relates generally to the extrusion of polymeric resins and like materials and, more particularly, to a crosshead and die assembly for the extrusion of single or multiple layers of thermoplastic materials. The invention is useful in the manufacture of concentric, tubular extrusions wherein uniform wall thickness is desirable, such as, for example, in the production of collapsible plastic squeeze tubes.
Heretofore, in the manufacture of continuous, concentric tubular extrusions, the thermoplastic resin, heated and under pressure from an extruder, enters the crosshead and die assembly and feeds a single resin distribution channel which is concentric, extending 360.degree. about an inner mandrel. The thermoplastic resin flows around and over the mandrel without any or very little control which, if not corrected or compensated, results in uneven wall thicknesses in the final tubular product. This condition creates unbalanced pressure and flow around the mandrel, making it necessary to compensate by choking-off an area of the resin flow with an outer die adjustment in order to obtain a uniform wall thickness. Needless to say, this requires close attention by the operator and careful control over the operating parameters of heat and pressure. In addition, in prior systems, the resin flowing over and around the inner mandrel, from a single entry point, must divide into separate streams and subsequently recombine. There is turbulence in the recombining resin streams which creates an objectionable condition on the finished tubular product. As the molten resin changes direction of flow of approximately 90.degree. in the crosshead, it also creates pockets of stagnated or nonflowing resin which causes limited special flow patterns and undesired channelling of the resin which disrupts the normally preferred laminar flow pattern. Such aberrations in the flow pattern of the resin carry through the crosshead and die assembly and appear in the extruded film or tubular shape or the like as a broad band or line commonly referred to as the weld line. In some cases, if the weld line is of sufficient intensity it may cause the tubular extrusion or other shape to rupture along the longitudinal axis of the extrusion. In addition, the outer surface of the extrusion in the area of the weld line is generally rougher than the balance of the extrusion periphery, which oftentimes presents cosmetic defects when the finished tube is to be decorated. In addition, when the thermoplastic resin is pigmented, the finished extrusion may also contain an area bordering the weld line of reduced or variable pigment concentration. This condition yields a nonuniform product which exhibits variant color striations in the finished tubing and may result in a quality rejection of the entire production run. Hence, it is readily appreciated that the crosshead and die devices commonly employed in the art have inherent shortcomings which require continuous and special operator attention in order to obtain uniform quality in the finished product.
The present invention is directed to a new and improved crosshead and die assembly which overcomes the problems heretofore encountered and provides an apparatus which produces balanced resin flow around the inner mandrel and is suitable for use in single layer as well as multi-layer extrusion operations for forming not only tubular products, but other formed or shaped extrusions such as flat sheet or folded tubular shapes. Products produced from the crosshead and die assembly of the present invention exhibit superior properties over those produced by known extrusion tooling while requiring less operator attention.